The invention relates to apparatus for performing optical measurements on surfaces that are obscured by debris that makes optical measurement difficult.
A wafer surface undergoing chemical mechanical polishing (CMP) is an example of a sample that will typically be obscured by debris that hinders optical measurements of the sample. The wafer is polished by rubbing the wafer between a wafer carrier and pad that is atop a platen. A slurry is typically used to mechanically and chemically facilitate removal of a portion of a film deposed on the wafer""s surface. The CMP slurry and residues adjacent to the wafer surface are typically optically inhomogeneous and opaque.
This debris (e.g., slurry and film residue) typically interferes with measurements of the sample. In a polishing process, it is desirable to detect when a film has been partially or completely removed from the wafer. When the film is partially or completely removed, this is usually referred to as the endpoint. It is important to detect the endpoint so that the wafer is not over polished. For example, in copper CMP, the copper film is initially optically opaque. Three endpoints are detected in copper CMP. First, it is determined when polishing of the copper has begun. When the copper film begins to become transparent, this transparency indicates that polishing has started. Second, it is determined when the copper is completely removed so that the underlying liner layer (e.g. Ta, TaN or WN) is exposed. Finally, it is determined when the liner layer has been removed.
When the endpoint of a film is reached, the polishing can then be stopped without polishing away other structures on the wafer. Since there is a lot of debris (e.g., slurry and/or film residue) associated with the CMP process, it would be difficult to accurately measure the endpoint while the wafer is undergoing CMP.
Various approaches to performing in situ optical measurement during CMP have been proposed. However, none of these approaches solve the problem of debris obscuring the wafer. Of note, U.S. Pat. No. 5,433,651 describes a single beam reflectometer employing a window within a cavity of the CMP polishing pad and platen. The described approach has the disadvantage that CMP slurry and residue can build up in the cavity formed within the platen/polishing pad. The slurry and residue make optical measurements difficult. Another approach, described in E.P. Patent 96302176.1, attempts to solve this problem by providing a xe2x80x9csoft windowxe2x80x9d within the cavity where slurry and residue might otherwise accumulate. Unfortunately, this window typically becomes scratched during the polishing process and pad conditioning and thereby also degrades the quality of optical measurements. Also, the material that is used to form the soft window typically scatters the measuring beam.
U.S. Pat. No. 5,081,796 describes moving a small edge portion of the wafer off the edge of the polishing pad, where the removed portion is then exposed to a jet of water which helps guide a beam onto the wafer""s edge. However, this approach has the disadvantage of only measuring the film at the edge of the wafer. Since only a small portion of the entire wafer surface is measured, measurement of the endpoint is not very accurate. Furthermore, this procedure may adversely affect the polishing process.
Thus, measurement apparatus and techniques for efficiently and reliably taking optical measurements of samples having associated debris that hinders measurements are needed. More specifically, mechanisms for efficiently removing debris from the measurement path while performing optical measurements are needed.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for measuring optical characteristics of a sample while clearing debris from the viewing area that is adjacent to the sample. As a result of clearing away debris, the sample may be optically monitored without debris significantly obstructing the optical path. In other words, a measurement beam may be directed towards a sample without being appreciably distorted by debris that may be present adjacent to the sample before and during the measurement process.
In one embodiment, a self-clearing objective for directing a beam towards a sample is disclosed. The objective includes an optical element arranged to direct the beam towards the sample and a substantially transparent fluid flowing between the optical element and the sample such that at least a portion adjacent to the sample is substantially cleared of debris. The fluid also directs the beam towards the sample.
In an alternative embodiment, the optical element and fluid are arranged to cooperatively direct the beam towards the sample. In one embodiment, the fluid is water or a gas. In yet another aspect of the invention, the optical element is in the form of a lens, a system of lenses, a fiber, a fiber bundle, a beam divider, a beam splitter, a beam collimator, a beam polarizer, a wave plate, or any combination thereof.
In another embodiment, the self-clearing objective described above is integrated within a reflectometer device for measuring a reflectivity value of the sample. The reflectometer device includes the self-clearing objective, a beam source arranged to generate the beam and direct the beam through the optical element, through the fluid of the self-clearing objective, and to the sample, and a detector arranged to receive a second beam from the sample in response to the first beam, and analyze the second beam to determine the reflectivity value of the sample.
In another embodiment, the self-clearing objective described above is integrated within an interferometer device. The interferometer device includes the self-clearing objective, a beam source arranged to generate the first beam and a reference beam and direct the first beam through the optical element, through the fluid of the self-clearing objective, and to the sample, and a detector arranged to receive a signal that is a result of interference between the first beam and the reference beam and to analyze the signal to determine a thickness of a film of the sample.
In another embodiment, the self-clearing objective described above is integrated within an ellipsometer device for measuring ellipsometric characteristics of the sample. The ellipsometer device includes the self-clearing objective, a beam source arranged to generate the beam and direct the beam through the optical element, through the fluid of the self-clearing objective, and to the sample, and a detector arranged to receive a second beam from the sample in response to the first beam, and analyze the second beam to determine the ellipsometric characteristics of the sample.
In yet another embodiment, the self-clearing objective described above is integrated within a photoacoustic device for measuring photoacoustic characteristics of the sample. The photoacoustic device includes the self-clearing objective as describe above, as well as a beam source arranged to generate the beam and direct the beam through the optical element, through the fluid of the self-clearing objective, and to the sample, and a detector arranged to receive a second beam from the sample in response to the first beam, and analyze the second beam to determine the photoacoustic characteristics of the sample. In a preferred embodiment, the detector is further arranged to determine a thickness of films within an opaque film stack deposited on the sample based on the measured photoacoustic characteristics.
In another aspect, the invention pertains to an in-situ chemical-mechanical polishing (CMP) apparatus for polishing a sample with a polishing agent and monitoring the sample. The CMP apparatus includes a polishing table having a viewing opening and a sample mount arranged to hold the sample over the polishing table. The polishing table and sample mount are arranged to receive a polishing agent between the sample and the polishing table and to polish the sample by moving the polishing table and the sample mount relative to each other. The CMP apparatus also includes an optical element proximate the viewing opening. The optical element is configured to receive an optical signal and direct it through the viewing opening to the sample. The CMP apparatus further includes a fluid conduit arranged to receive a fluid between the optical element and the sample to thereby clear away a portion of the polishing agent within the viewing opening such that the optical signal is not substantially altered by the polishing agent before the optical signal reaches the sample.
In a preferred embodiment, the pressurized flow of the fluid is configurable to substantially remove the polishing agent from the viewing opening. In another embodiment, the CMP apparatus also includes a measurement device arranged to detect an endpoint of a film of the sample. For example, the measurement device is either a reflectometer, an ellipsometer, interferometer, or a photoacoustic detector.
The present invention provides several advantages. For example, the self-clearing objective allows a sample to be measured while undergoing a process that may leave debris adjacent to the sample or within the measurement path. This greatly increasing the efficiency of the process since the sample does not have to be removed from the process to be cleaned and then measured outside of the process. Additionally, since the self-clearing objective includes a flowing fluid, it is unlikely to be damaged by an abrasive process, such as CMP. In contrast, a conventional xe2x80x9chard or soft windowxe2x80x9d is likely to be damaged by an abrasive process, such as CMP (e.g., the window is scratched by the slurry and/or pad conditioning). Additionally, the self-clearing objective does not accumulate scratches that may themselves obscure the sample by scattering the measuring beam, as compared to scratches on a conventional soft or hard window that tend to scatter the measuring beam.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.